This invention relates to a process and catalyst for the oxidation of hydrocarbons and, in one preferred embodiment, for selective production of alkanes that are terminally substituted with oxygenated moieties, for example, to the production of aldehydes, 1-alkanols and/or 1-carboxylic acids from the corresponding alkane, most preferably straight-chain alkane.
Oxidation of hydrocarbons to produce oxygenated products has been carried out using a number of techniques, including the use of various catalysts and oxidizing agents. Typically, however, oxidation of straight chain alkanes produces a mixture of oxygenated products, with the oxidation taking place predominantly at non-terminal carbon atoms in the chain. For instance, oxidation of n-hexane typically primarily produces a mixture of 2- and 3-hexanols, together with the corresponding ketones. Aliphatic aldehydes, such as valeraldehyde (pentanal), caproaldehyde (hexanal), enanthaldehyde (heptanal), caprylaldehyde (ocatanal), capraldehyde (decanal), and the like, are typically produced, on the other hand, by hydroformylation of an olefin, i.e. by reacting an olefin having one less carbon atom than the desired aldehyde with hydrogen and carbon monoxide in the presence of a suitable catalyst. However, these processes are expensive, involving complicated chemistry and expensive feeds. In addition, hydroformylation of olefins first requires oligomerization to form the longer chain olefins. Primary alcohols are typically currently produced by olefin hydroformylation, ester hydrogenation or olefin hydration using hydrogen peroxide and boric acid (see, e.g., Zweifel et al., J. Am. Chem. Soc. 89, 291 (1967).
Selective oxidation of alkanes, especially straight-chain alkanes, is a less than straightforward operation. Enzymes (e.g. ω-hydroxylase) with non-heme iron active centers have been found to catalyze the oxidation of alkanes using O2 with high terminal regioselectivity, apparently because proteins near active centers lead to selective docking and binding. See, for instance, Hamberg et al., in Molecular Mechanisms of Oxygen Activation (ed. Hayaishi, O.) 24-52 (Academic Press, New York, 1974). Many recent studies have attempted to mimic these unique properties. About 20% terminal regioselectivities were reported for linear alkanes on sterically-hindered Mn(III) active centers in metalloporphyrins [Cook et al., J. Am. Chem. Soc. 108, 7281 (1986)]. As described in that publication, n-hexane oxidation by iodosobenzene (PhIO) oxidant on 5,10,15,20-tetrakis(2′,4′,6′-triphenylphenyl)-porphyrinato-manganese(III)acetate [(MnTTPPP(OAc)] catalysts gave 19% 1-hexanol among all hexyl-alcohols. The corresponding primary selectivity index (kprim/ksec; defined as the ratio of primary to secondary products normalized by the number of each type of C—H bonds) was 0.31. Inorganic solids that catalyze oxidation of alkanes to alcohols, ketones, and acids using O2 remain a significant challenge.
Some research has been carried out on oxidation of alkanes using zeolites containing metallic components. Regioselective oxidation to primary substituted oxygenated compounds was achieved in some cases. For instance, Tatsumi et al., American Chemical Society Symposium Series 638:374 (1996) and Res. Chem. Intermed. 24:529 (1998) investigated oxidation of n-hexane and cyclohexane with the vanadium-containing zeolite VS-2 using hydrogen peroxide as the oxidant, and found that the catalyst was suitable both for oxidation of n-hexane, with some selectivity towards terminally substituted compounds, and for oxidation of cyclohexane. This was contrasted by the authors, however, to previous work by others on titanium zeolite analogs TS-1 and TS-2, which gave only secondary alcohols and ketones from the oxidation of n-hexane using hydrogen peroxide.
Herron et al. [J. Am. Chem. Soc. 109:2837 (1987)] found appreciable terminal oxidation of n-pentane, n-octane and n-decane using zeolite 5A ion exchanged with iron and with palladium. Here the oxidant was a mixture of hydrogen and oxygen, which formed hydrogen peroxide in situ. Some work was also carried out with similar catalysts using zeolite ZSM-5. Herron [New J. Chem. 13:761 (1989)] demonstrated good results of this type using a ZSM-5 zeolite containing only iron, with hydrogen peroxide as the oxidant. Tolman et al. [Proc. Ann. IUCCP Symposium (1987); Martell, ed.] also conducted work using a mixture of hydrogen and oxygen as the oxidant. Thomas et al. [Nature, 398, 227 (1999)] describe oxidation of n-hexane and n-octane with dry air using aluminophosphate molecular sieves [AlPO materials] conaining Co+3 or Mn+3 ions as part of the framework. The authors claimed to have achieved as high as 60+% terminal oxidation products (alcohol, aldehyde, and carboxylic acid combined, with the acid predominating) from n-hexane and n-octane with one of their catalysts, MnAlPO18 (which has 8-ring open windows).
Demonceau et al. [J. Molec. Catal. 49, L13 (1988)] found that homogeneous sterically-hindered Rh 2,4-dichloro-3,5-dinitrobenzoic carboxylate complexes gave modest terminal regioselectivities (31%, kprim/ksec=0.60) for carbene insertion into C—H bonds in n-hexane. Recently, terminal regioselectivity was also reported in borylation of saturated alkanes, in which Bis(pinacolato)-diborane (B2pin2) or pinacolborane (HBpin) with some specificity with terminal C—H bonds in alkanes on Rh complexes with bulky ligands (Cp*Rh(η4-C6Me6)) [Chen et al., Science 287, 1995 (2000)]. For example, n-octane reactions with HBpin led to n-octyl-1-Bpin, which was obtained with 65% yield after reaction for 14 hours.